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I live in S. Florida and want install transfer grills above the doors of our bedrooms to increase the return air flow back to the main return grill in the livingroom. Currently the doors are slightly cut on the bottom but I don't believe it provides the proper amount of return air.

I would like to know how these are typically installed.
I will be cutting a how in the sheetrock on each side of the wall above the door and will place about a 6"X12" grill on each side.

I need to know a few things:

If there is a stud which obstructs air flow; is it cut and braced horizontally or just left in place and ignored.

To keep air from flowing from the insides of the wall, should I frame it out with wood or just try to seal it off with pieces of duct board and foil tape?

Will I need a baffle between the grills to reduce noise and retain privacy between rooms? If so, can I buy these pre-made or is the an easy way to make them myself.

I have not seen a home yet with a return duct system in South florida; is this common practice?

Do you think adding the transfer grills is overkill and I should just undercut the doors more?

If you have access to the ceiling, a jump duct made from two registers connected by flex duct will minimize noise transfer. They do make transfer grilles designed to minimize noise and/or fire transfer between rooms.

Feel the air flow to the room,at the grille,then have someone close the door,the reduction in air flow ,is the difference you will get.

Size the transfer grille larger than the supply grille,even larger if you hit a stud.Wall cavity should be sealed ,as it's likely has small openings to the attic,ductboard and mastic is what we use.

Jumper ducts ,as mentioned transfer much less noise,severla rooms can go to a larger grille in the hall.Baffles in transfer grilles will mrestrict airflow,though nthere is a product made for this,grille would need to be larger.